The root is that online speech is 100% privatized. There is no online post office whose only rules are national law and is completely immune from public backlas

The root is that online speech is 100% privatized. There is no online post office whose only rules are national law and is completely immune from public backlash against its users. Nobody pickets the US post office when Nazis use it to mail hateful newsletters to each other. Online providers don't have that immunity. Everything ever said by their users has the potential to severely impact their financial health if the public reaction is significant enough.